A transversal filter is an electronic device that accepts input information in the form of an analog electrical signal and processes the input information according to a set of electrical signals applied to a second input, referred to as a programming or reference input. The processed information is supplied as an analog electrical output signal.
The ACT programmable transversal filter (PTF) is an ACT integrated circuit based high speed analog signal processing device which provides the capability to realize arbitrary transversal filter impulse responses. This is achieved through a device architecture which permits independent digital programming of each of the tap weights in an ACT transversal filter.
A conventional transversal filter has a single delay line which is periodically tapped to provide the required incrementally delayed replicas of the input signal. Prior transversal filters have utilized conventional surface acoustic wave ("SAW") structures, or charge coupled devices ("CCD") as the delay line. The delayed replicas are multiplied by predetermined coefficients, known as tap weights, and the products are summed to form the output signal. The tap weights may be determined by fixed device features so that they are permanently established, or they may be programmable in order to permit the filter response to be determined dynamically by electronic means.
Attempts have been made to combine surface acoustic wave or charge coupled device delay lines with electronic circuitry necessary for loading and storing the tap weight reference signals, as well as the circuitry necessary to permit the tap weight signals to be multiplied by the delayed input signal replicas. The surface acoustic wave devices have, in the past, been hybrid combinations of one type or another. The primary difficulty in hybrid approaches, however, involves difficulty in manufacturing, signal frequency bandwidth limitations and tap weight control speed degradation caused by the interconnecting bond requirements between either the delay line taps and the multiplying circuit, or between the multiplying circuit and the tap weight storage cells.
An acoustic charge transport device ("ACT device") is a novel analog semiconductor device which is particularly useful as a delay line, and for performing various signal processing functions. The ACT device overcomes many of the problems of the prior charge coupled devices, because the ACT device processes signals at extremely high speeds while avoiding the necessity of clock driver circuitry. The ACT device achieves its high processing speed because a surface acoustic wave propagates through a channel buried within the piezoelectric semiconductor substrate, and the wave transports the charge, representative of the input signal, through the channel at the wave speed. The charge is transported in discrete identifiable packets. Transportation of charge through the buried channel produces an output at the drain (output contact) which is in the form of sharp current pulses.
The voltage-to-charge conversion process occurring at the ACT device input (input contact) is one wherein the amount of charge injected into each packet is proportional to the input voltage at specific sample points in time. In the ACT device, these sample points tend to occur at the instants in time when the positive peaks of the potential are under the input contact. Hence, the input contact of the ACT device operates as an automatic time domain sampler, having a sampling rate equal to the frequency of the propagating surface acoustic wave. The ACT device has very broad band characteristics enabling the device to process signals from baseband to many hundreds of megahertz. In addition, the ACT device utilizes a GaAs substrate which permits the use of high speed circuits for the implementation of additional high frequency signal processing functions, such as RF signal gain blocks.
Those skilled in the art will understand that there is a need for programmable transversal filter which avoids the interconnecting bond limitations of prior surface acoustic wave technology, and which furthermore overcomes the bandwidth limitations attributable to charge coupled device technology. The disclosed invention provides just such a novel programmable transversal filter by utilizing an ACT device in a manner which permits the delay, multiplication, summation and storage functions to be done in close proximity on the same substrate, in order to avoid bond wire interconnections while also advantageously utilizing the broad bandwidth characteristics of an ACT device.